The scale on a map is 2cm : 1km. If the distance between two cities is 2km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 2cm on the map corresponds to an actual distance of 1km. An actual distance of 2km is the same as 2 $\cdot$ 1km. The distance between the two cities on the map is 2 $\cdot$ 2cm, or 4cm.